Add justification for RFU to comments

src/policy/value.h

@@ -8,7 +8,13 @@
 
 #include <cstdint>
 
-/** Amount denominated in the node's RFU (reference fee unit) */
+/** ELEMENTS: Amount denominated in the node's RFU (reference fee unit). Used only
+ *  when con_any_asset_fees is enabled in order to distinguish from amounts in an 
+ *  actual asset. RFU is needed to make amounts comparable when sorting transactions 
+ *  in the mempool, as well as for fee estimation and subsequent validation of those 
+ *  fees according to various limits (e.g., mintxfee, paytsxfee, blockmintxfee, 
+ *  incrementalrelaytxfee, etc.).
+ */
 struct CValue 
 { 
     int64_t value;